Fifth wheels of the type used to detachably couple the forward end of a semi-trailer to a tractor generally are attached to rail members rigidly secured to the tractor frame. This results in a rigid interconnection between the tractor frame and the forward end of the trailer. This results in the transmission of substantial shock between the trailer and tractor through the fifth wheel assembly. As a consequence, loads within the trailer, particularly at its forward end, often are subjected to damage when the wheels of the tractor and trailer pass over bumps or pot holes in the road. Also, the tractor itself is subjected to incessant pounding from the trailer through the rigid fifth wheel coupling. Thirdly, the tractor driver is subjected to this pounding or shock throughout the time he is operating the vehicle creating fatigue and often serious physical problems.
To alleviate the problems resulting from the incessant pounding of the tractor by the trailer and vice-versa through the fifth wheel assembly, various proposals have been made in the past. For the protection of the driver, the driver's seat is mounted with resilient suspension between it and the tractor cab. An attempt illustrated in Gouirand U.S. Pat. No. 2,968,496 to isolate the tractor frame from the trailer has been made by mounting the fifth wheel assembly on the tractor through a special suspension system directly supported by the rear axle of the tractor instead of the frame. Such departure from a standard fifth wheel mounting however requires expensive and complex rebuilding or customized construction of the tractor rear axle and has not resulted in any significant acceptance in the industry.
Other proposals have been made for providing a resilient mounting for the fifth wheel mounting plate in the form of a support attached across the top of the tractor frame. Various types of resilient steel springs and pneumatic air bag springs have been proposed for this purpose, but the resultant structure raises the forward end of the trailer to generally unacceptable heights. Even if the suspension system is capable of being lowered to a non-support position (particularly in the case of airbag suspensions), the relatively bulky structure which must be mounted on top of the tractor frame still prevents the lowering of the forward end of the trailer sufficiently for many uses. As a result, the operator of a tractor equipped with such a prior art fifth wheel assembly is given two choices. Either he must restrict his trailer height to a height which is less than he could use with a conventional fifth wheel assembly, or the tractor trailer must be routed, often over relatively long detours, around any overhead obstructions which do not provide sufficient clearance for the elevated trailer. In most cases both of these alternatives are unacceptable. Examples of fifth wheel mounting assemblies of this type are shown in the patents to Masser, U.S. Pat. No. 3,137,515 issued June 16, 1964; Granning, U.S. Pat. No. 3,380,758 issued Apr. 30, 1968; Chalmers, U.S. Pat. No. 2,821,409 issued Jan. 28, 1958; and Granning, U.S. Pat. No. 3,253,840 issued May 31, 1966. For the most part the disadvantages of these systems created because of their relatively complex nature and the additional height which is added to the trailer under all conditions of use, have resulted in minimal acceptance of such assemblies.
It is desirable to provide a fifth wheel mounting assembly which effectively cushions and isolates the tractor and trailer from communicating shock during operation of the tractor trailer and which is of simple construction so that it can be incorporated as original equipment onto a tractor or readily mounted onto an existing tractor with minimum effort. In addition it is desirable to include a resilient mounting for fifth wheel of a tractor trailer combination which does not significantly increase the height of the trailer when the mounting assembly is placed in its lowermost position.